Lymphatic filariasis is the most common cause of acquired lymphedema worldwide (Szuba and Rockson, 1998). It is endemic to tropical and subtropical regions, and its effects are devastating. With over 100 million infected persons, it ranks second only to leprosy as the leading cause of permanent and long-term disability. Wuchereria bancrofti is the etiologic agent in 90% of cases. There is a dearth of published MRI findings with pathologically proven active infections, making this entity even more of a diagnostic dilemma. Imaging may provide the first clue that one is dealing with a parasite and may facilitate proper treatment and containment of this disease. This is the first report of pathologic correlation with MRI findings in the extremity in active filariasis. The magnetic resonance images demonstrate an enhancing, infiltrative, mass-like appearance with partial encasement of vasculature that has not been previously described in filariasis. Low signal strands in T2-hyperintense dilated lymphatic channels are seen and may depict live adult worms. We hypothesize that the low signal strands correspond to the collagen rich acellular cuticle. This, in combination with the surrounding hyperintense T2 signal, corresponding to a dilated lymphatic channel, may provide more specific MRI findings for active nematodal infection, which can prompt early biopsy, pathological correlation, and diagnosis. 1. Case Report A 33-year-old male from Nepal, who immigrated to the United States 3 years ago, presented to the Emergency Department with pain and redness at his right mid-arm for 10 days. He did not appear ill and had no fever or weight loss. The area was red, swollen, and tender. There was also enlargement of axillary lymph nodes. MRI demonstrated the presence of an enhancing soft tissue mass with infiltrative features, partially encasing the brachial vessels, in addition to the axillary lymphadenopathy (Figure 1). Foci of low signal intensity were also noted on both T2 and T1 weighted images (Figure 1). Figure 1: (a) Coronal STIR MR image of the right upper arm (TR = 3100; TE = 62.24; FOV = 38？cm). There is increased T2 signal extending from the axilla along the medial soft tissues of the upper arm following the lymphatic structures and paralleling the neurovascular bundle. (b) Axial T2 TSFSE (TR = 3000; TE = 42.816; FOV = 16) at the level of the mid to distal humerus where focal soft tissue swelling is present. There is an irregular area of increased T2 signal medially containing punctate low signal foci. (c) Coronal STIR MR image (TR = 3100; TE = 62.24; FOV =
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